1. Field of the Invention
The present invention relates to a current collector containing copper, and an anode and a battery using it.
2. Description of the Related Art
In recent years, portable electronic devices such as combination cameras (videotape recorder), mobile phones, and notebook personal computers have been widely used, and it is strongly demanded to reduce their size and weight and to achieve their long life. Accordingly, as a power source for the portable electronic devices, a battery, in particular a light-weight secondary batter capable of providing a high energy density has been developed.
Specially, a secondary battery using insertion and extraction of lithium for charge and discharge reaction (so-called lithium ion secondary battery) is extremely prospective, since such a secondary battery provides a higher energy density compared to a lead battery and a nickel cadmium battery.
The lithium ion secondary battery has a cathode, an anode, and an electrolytic solution. The anode has a structure in which an active material layer is provided on a current collector. The active material layer contains an active material contributing to electrode reaction. As the active material of the anode, a carbon material has been widely used. However, in recent years, as the high performance and the multi functions of the portable electronic devices are developed, further improvement in the battery capacity is demanded. Thus, it has been considered to use silicon instead of the carbon material. Since the theoretical capacity of silicon (4199 mAh/g) is significantly higher than the theoretical capacity of graphite (372 mAh/g), it is prospected that the battery capacity is thereby highly improved.
However, when silicon as an active material is deposited by vapor-phase deposition method, the active material layer tends to be hardly contacted with the current collector. Thus, according to the contact degree, the active material layer may be separated from the current collector when charge and discharge are repeated. If the active material layer is separated, electrode reaction of the anode is inhibited by lowering of the current collection characteristics or the like. In the result, the cycle characteristics as important characteristics of secondary batteries are lowered.
Therefore, to improve the cycle characteristics even when silicon is used as the active material of the anode, various devices have been invented. Specifically, a technique using a current collector having an extension coefficient of 13% or more (for example, refer to Japanese Unexamined Patent Application Publication No. 2005-135856), a technique using a current collector having arithmetic average roughness Ra of from 0.01 μm to 1 μm (for example, refer to Japanese Patent No. 3733066), a technique using a current collector having a tensile strength of 3.82 N/mm or more and having arithmetic average roughness Ra of from 0.01 μm to 1 μm (for example, refer to International Publication No. WO 01/29912) and the like have been proposed.